The lowest detected stellar Fe abundance: the halo star SMSS J160540.18−144323.1

Abstract

We report the discovery of SMSS J160540.18−144323.1, a new ultra-metal poor halo star discovered with the SkyMapper telescope. We measure [Fe/H] = −6.2 ± 0.2 (1D LTE), the lowest ever detected abundance of iron in a star. The star is strongly carbon-enhanced, [C/Fe] = 3.9 ± 0.2, while other abundances are compatible with an α-enhanced solar-like pattern with [Ca/Fe] = 0.4 ± 0.2, [Mg/Fe] = 0.6 ± 0.2, [Ti/Fe] = 0.8 ± 0.2, and no significant s-or r-process enrichment, [Sr/Fe] < 0.2 and [Ba/Fe] < 1.0 (3σ limits). Population III stars exploding as fallback supernovae may explain both the strong carbon enhancement and the apparent lack of enhancement of odd-Z and neutron-capture element abundances. Grids of supernova models computed for metal-free progenitor stars yield good matches for stars of about 10 M ⊙ imparting a low kinetic energy on the supernova ejecta, while models for stars more massive than roughly 20 M ⊙ are incompatible with the observed abundance pattern.

The SkyMapper DR1.1 search for extremely metal-poor stars

“…A sixth star, SMSS J160540.18-144323.1 first identified in this work, has also been shown to have [Fe/H] < -5; seeNordlander et al (2019). 2 see http://skymapper.anu.edu.au 3Beers & Christlieb (2005) use the terminology 'extremely', 'ultra' and 'hyper' metal-poor to designate stars with [Fe/H] < -3.0, -4.0 and -5.0, respectively.…”

“…For example, the most metal-poor star in this sample, SMSS J160540.18-1414323.1, which is the blue filled square symbol in the bottom panel of Fig. 9 and which has one of the highest W (G) values, is shown in Nordlander et al (2019) to be strongly carbon-enhanced ([C/Fe]1D,LT E = 3.9 ± 0.2). The other two stars with [Fe/H] f itter below -4 (the red filled triangle and the magenta open circle in Fig.…”

“…16 What is clear from this analysis is that, in regard to Group III stars, because of the overall uncertainty in the v-band photometry and the small number of stars involved, we cannot make any strong statements about completeness from the current set of [Fe/H] f itter values, which are derived from the low resolution spectra of stars within the photometric selection window. We have revealed at least one new candidate (SMSS J160540.18-144323.1 with [Fe/H] < -4.75 and [C/Fe] likely exceeding +2; (see Nordlander et al (2019) for the actual values), but it is unclear how many Group III stars at [Fe/H] ≈ -4 or thereabouts have been overlooked because, as Fig. 19 shows, the strong CH-features do move the star downwards (particularly at redder colours) in the metallicity-sensitive diagram into the region where the fraction of candidates observed is low, and the 'contamination' from somewhat more metal-rich objects is large.…”

“…Table 2, and the online supplementary material, list the SkyMapper designation, J2000 position, g, (g − i)0 and metallicity index mi from the DR1.1 photometry, G-band (CH) equivalent width (see §3.4), and the spectrophotometric fit parameters for the 142 stars with [Fe/H] f itter < -3.0 dex. High dispersion spectroscopic follow-up is presented and discussed in Nordlander et al (2019) for the most metal-poor object SMSS J160540.18-144323.1, and in Yong et al (2019) for a large fraction of the other stars in Table 2 based on observations obtained with the MIKE echelle spectrograph on the Magellan Clay 6.5m telescope. For completeness we give in Table 3, and in the online supplementary material, the same information as for Table 2 for the 29 stars observed at the 2.3m which have [Fe/H] f itter -3.25 that either fail the DR1.1 photometric selection criteria and/or lie outside the adopted selection window.…”

A bound on the 12C/13C ratio in near-pristine gas with ESPRESSO

“…Bond 1980;Beers et al 1985Beers et al , 1992McWilliam et al 1995;Cayrel et al 2004;Beers & Carollo 2008;Howes et al 2016) have revealed an interesting trend in the chemical composition of the lowest metallicity stars. Notably, there is an overabundance of carbon in some of the most iron-poor stars found in the halo of the Milky Way; indeed, every star with a measured iron abundance [Fe/H] ≤ −5.0 exhibits a strong carbon enhancement 1 (Christlieb et al 2004;Frebel et al 2005;Aoki et al 2006;Frebel et al 2015;Allende Prieto et al 2015;Nordlander et al 2019). Despite concentrated efforts, and increasingly sophisticated cosmological hydrodynamic simulations, we have yet to establish whether or not low mass (∼ 1 M ) metal-free stars can form.…”